Method of producing silver-cadmium oxide electrical contact elements



y 1966 c. B. GWYN, JR 3,258,829

METHOD OF PRODUCING SILVER-CADMIUM OXIDE ELECTRICAL CONTACT ELEMENTS Filed July 12, 1963 13 5-1- mid- United States Patent 3,258,829 METHOD OF PRODUCING SILVER-CADMIUM OXIDE ELECTRICAL CONTACT ELEMENTS Childress B. Gwyn, J12, Export, Pa., assignor, by mesne assignments, to Talon, Inc., a corporation of Pennsylvama Filed July 12, 1963, Ser. No. 294,635 8 Claims. (Cl. 29155.55)

This invention relates to a method for producing silvercadmium oxide electrical contact elements, and more particularly relates to a method for producing such elements by so-called cold working or cold heading techniques.

It is known to utilize silver-cadmium oxide contact materials for many electrical contact applications. Conventionally, such contacts may be fabricated, employing known powder metallurgy techniques, by initially blending silver and cadmium oxide powder constituents, pressing the mixture in a confining die at suitable pressures, generally with one or more subsequent heating or sintering operations and, frequently, thereafter coining or re-pressing the sintered compacted silver-cadmium oxide body by so-called used press-repress operations. For subsequent cold heading into rivets or other shapes for electrical contact elements, the silver-cadmium oxide bodies thus produced must be further worked into suitable forms or blanks, e.g., having the shape of a wire.

The fabrication properties of materials of this type are difiicult to duplicate in successive experiments and extrusion, swaging or drawing of such compositions into wire form is extremely difficult. It has been found that small incipient surface cracks, or longitudinal cracks parallel to the lines of work force, produce strain lines or local formative bodies in the wire or other blanks, which complicate fabrication of electrical contacts therefrom. Moreover, contact rivets formed from such blanks are frequently subject to fracture, both during cold heading or other forming operations and when subjected to subsequent riveting operations. Frequently, when such compacted silver-cadmium oxide powder mixtures are formed into desired contact element shapes, scalloped edges (edge cracks) or dropped-out particles are formed; such malformations may result in early failure of the resulting electrical contact when subjected to the electrical and mechanical stresses of operation.

It is also known to employ silver-cadmium alloy bodies, which may include minor percentages of nickel, cobalt, copper, zinc, lead, etc., in wire or sheet form to facilitate fabrication of contact element blanks. Such blanks may thereafter be internally oxidized to silver-cadmium oxide subsequent to cold heading or working. The wire so produced has proven generally superior for cold heading purposes than that produced from compacted silver-cadmium oxide powder compositions for the same purpose.

However, it has been found that the known internal oxidation processes frequently do not achieve complete oxidation throughout the silver-cadmium alloy bodies treated, but rather produce heterogeneous bodies having external silver-cadmium oxide surfaces and only partially or negligibly oxidized alloy materials extending inwardly of such surfaces. When such an internally oxidized alloy material is utilized in wire form for forming an electrical contact strip or rivet by a cold heading procedure, the originally non-oxidized or only partially oxidized central portion of the blank may be spread over a considerable area of the working face of the contact element, leading to possible early failure or faulty operation when later subjected to mechanical and electrical stress. Furthermore, edge-cracking and/ or particle drop-out of the internally oxidized silver-cadmium alloy elements frequently occurs in the same manner as occasioned by the use of contact elements constituted of compacted silver-cadmium oxide mixtures.

It is desirable to utilize silver-cadmium oxide contact elements having relatively high proportions of cadmium oxide, viz., in excess of 10% by weight of the total composition. In some cases optimum service characteristics are obtained employing contact bodies constituted of by weight silver and 20% by weight of cadmium oxide; such a body is extremely difficult to produce in wire form from powdered mixtures of silver and cadmium oxide, and virtually impossible to produce by oxidation of an appropriate silver-cadmium metal alloy since the time-temperature cycle required for oxidation of such an alloy is impractical. Moreover, such silver-cadmium oxide bodies, even when formed, are exceedingly difficult to fabricate into contact elements by cold heading operations.

In is accordingly among the objects of the present invention to provide a method for simply and economically producing silver-cadmium oxide electrical contact elements without the concomitant production of substantial mechanical weaknesses.

Another object of the invention is to provide such a method which may be utilized to form contact elements containing relatively high proportions of cadmium oxide, i.e., in excess of 10% by weight of the composition, which elements may be readily fabricated to produce electrical contacts having optimum service characteristics.

The nature and objects of the present invention will be more fully apparent from a consideration of the following detailed description of a preferred embodiment thereof, taken in connection with the accompanying drawing in which FIGURES 1 and 1a are a side elevation and end view, respectively, of a silver-cadmium oxide contact body to be formed into a contact rivet in accordance with the present invention;

FIGURES 2 and 3 are vertical cross-sections through, and FIGURES 2a and 3a corresponding horizontal crosssections, of the contact body during successive stages of its formation into the desired contact rivet; and

FIGURES 4 and 4a are a side elevation and end view, respectively, of the final contact rivet element formed by the method of the invention.

Generally, the herein involved method of producing a silver-cadmium oxide contact element involves forming a malleable external surface sheathing of a silver-cadmium alloy on a silver-cadmium oxide core, cold working the resulting blank into the configuration of the desired contact element and thereafter oxidizing at least the portion of the surface sheathing on the working surface of the contact element to silver-cadmium oxide.

By cold heading or forming a blank having a silvercadmium alloy outer sheathing and thereafter oxidizing at least the portion of the sheathing to be utilized as the working surface of the electrical contact, much of the edge cracking, scalloping and fracturing of the contact element during or after cold forming is eliminated. Moreover, while the maximum cold flow (upsetting) ratio obtainable during cold heading operations employing compacted silver-cadmium oxide powder metal bodies is only about 2 or 2.5 to 1, when the procedure of the invention is employed, the silver-cadmium alloy sheathed blanks may be subjected to cold flow ratios of as much as 4 to 1 or more without the formation of substantial defects in the product contact elements.

The silver-cadmium alloy external surface sheathing may be produced by applying a silver-cadmium alloy cladding over a silver-cadmium oxide core or, alternatively, by reducing the external surface of a silver-cadmium oxide element by treatment with a reducing vapor,

e.g., hydrogen, combusted city gas (CO) or nitrogen, at a temperature of from 300 to 800 C. for a period of from to 240 minutes, to reduce the external surfaces to the silver-cadmium alloy. The external surface sheathing may also be produced by treating the surface of a silvercadmium oxide element with a suitable acid, ammonium nitrate, ammonium hydroxide, or the like, to leach out or convert the oxide to the desired silver-cadmium alloy. It will of course be understood that unalloyed silver and cadmium may additionally be present in the surface sheathings thus provided.

In any case it is preferred that the initial silver-cadmium body comprise from about 95 to 80 weight percent silver and from about 5 to 20 weight percent cadmium oxide, and that the silver-cadmium alloy sheathing formed thereon comprise from about 96 to 78 weight percent silver and from about 4 to 22 weight percent cadmium. The sheathing should comprise only a minor proportion of the overall thickness of the blanks subjected to cold heading; hence in the case of cylindrically shaped blanks utilized for the formation of rivet contact elements, it is preferred to provide the external surface sheathing with an annular thickness of from about 0.001 to 0.003", no more than about 5% of the over-all diameter of the cylindrical blank.

The sheathed blank is thereafter cold formed into the configuration of the desired contact element, e.g., a contact rivet, and the surface sheathing on at least the working surface or face of such element oxidized to silvercadmium oxide. The last mentioned oxidation step may be conveniently performed by heating the contact element at a temperature of from 600 to 850 C. for a period of from 5 to 240 minutes in the presence of atmospheric oxygen or other oxygen-containing gas.

It has been found that cracking or scalloping of the composite blank during the cold heading operation is substantially reduced by abrading the surface sheathing thereof at right angles to the length of the blank as initially formed. The abrading operation may be performed by either a centerless grinding or Wire brushing of the external sheathing of the wire or blank. While the explanation for the improved results obtained by the abrading procedure is not completely understood, it is believed that the abrading effects removal of loose surface flakes and particles which would otherwise be subsequently incorporated in the surface or body of the contact element produced by cold heading, relieves surface strains and smoothes over or closes any cracks or fissures formed during processing of the blank prior to the upsetting or cold forming operation.

Turning to the drawing, as shown in FIGURES 1 and 1a, a cylindrical wire shaped silver-cadmium oxide body 11 constituted of 80 weight percent silver and 20 Weight percent cadmium oxide is initially provided. In the embodiment illustrated, the body 11 is first exposed to a reducing atmosphere at a temperature of from 300 to 800 C. for a period of from 5 to 180 minutes to reduce the surface layer 12 of the element 11 to the corresponding silvercadmiurn metal alloy comprising about 75 to 95 weight percent silver and about 5 to 25 weight percent cadmium. In one case the surface of a wire body 11 having a 0.1" diameter was reduced to form a silver-cadmium alloy surface sheathing having an annular thickness of 0.005", about a 0.09" thick silver-cadmium oxide core.

As noted above, the composite wire blank thus produced (FIGURES 2, 2a) may instead be formed by cladding a silver-cadmium oxide core with a suitable silver- 'cadmium alloy sheathing by conventional cladding techniques.

Preferably, the composite sheathed contact blank is thereafter abraded as described above, and then subjected to a cold forming operation to produce the rivet contact element shown in FIGURES 3 and 3a, in which the sheathing 12' of the contact rivet covers the shank 14 and a portion of the head 15 of the rivet. The core 13 of the rivet is composed of the silver-cadmium oxide material extending to and covering the principal portion of the working surface 16 of the rivet.

The contact rivet is thereafter subjected to oxidation, e.g., by exposure to an oxygen containing atmosphere, at a temperature of from 600 to 850 C. for a period of from 5 to 240 minutes, to re-oxidize the silver-cadmium alloy sheathing 12' to silver cadmium oxide. A homogeneous rivet 17 is thereby produced (FIGURES 4, 4a) comprised of the desired silver-cadmium oxide mixture. It will, however, be understood that a portion of the sheathing 12' may remain on the rivet 17, it only being necessary that at least the portion of the sheathing on the working surface 16 of the contact rivet be oxidized to silver-cadmium oxide to provide the desired electrical properties for the facing material of the contact.

Accordingly, a method has been provided in accordance with the invention for producing silver-cadmium oxide electrical contact elements which are substantially free of deleterious contaminants and surface imperfections and provide excellent electro-mechanical strength and long service life. Since certain changes may be made in carrying out the above method without departing from the scope of the invention, it is intended that all matter con-' tained in the above description shall be interpreted as illustrative and not in a limiting sense.

The embodiments of the invention in which an exclusive privilege or property is claimed are defined as follows:

1. A method of producing a silver-cadmium oxide electrical contact element, comprising forming a malleable external surface sheathing constituted of a silver-cadmium alloy on a silver-cadium oxide core, cold working the resulting blank into the configuration of the desired contact element and oxidizing at least the portion of said sheathing on the working surface of the contact element to silver-cadmium oxide.

2. The method as defined in claim 1, in which said sheathing is produced by reducing a surface layer of a silver-cadmium oxide core element to a silver-cadmium alloy in a reducing atmosphere at a temperature of from 300 to 800 C. for a period of from 5 to 240 minutes.

3. The method as defined in claim 1, in which said sheathing is produced by cladding a silver-cadmium oxide core element with an outer layer constituted of a silvercadmi-um alloy.

4. The method as defined in claim 1, including the step of abrading the surface of said blank, prior to cold working the same, at right angles to the direction in which said blank has initially been formed.

5. A method of producing a silver-cadmium oxide electrical contact rivet, comprising heating the surface of a cylindrical silver-cadmium oxide body at a temperature of from 300 to 800 C. in a reducing atmosphere for a period of from 5 to 240 minutes to reduce the surface of said body to a silver-cadmium alloy defining a malleable external sheathing on said body, abrading the surface of the sheathed body at right angles to the length thereof, subjecting the thus treated body to axial pressure to form the same into a contact rivet element, the working surface of the head of which rivet is constituted principally of silver-cadmium oxide, and heating said rivet element at a temperature of from 600 to 850 C. for a period of from 5 to 240 minutes in the presence of atmospheric oxygen to oxidize at least the portion of the silvercadmium alloy sheathing on said working surface to silver-cadmium oxide.

6. The method as defined in claim 5, in which the initial cylindrical silver-cadmium oxide body is constituted of a mixture of from 75 to weight percent silver and from 5 to 25 weight percent cadmium oxide, and in which the silver-cadmium alloy surface sheathing formed thereon is constituted of from 75 to 95 weight percent silver and from 5 to 25 weight percent cadmium and has a thickness of no more than 5% of the over-all thickness of the resulting blank.

7. A method of producing a silver-cadmium oxide electrical contact rivet, comprising applying a silver-cadmium alloy cladding to a cylindrical core constituted of silvercadmium oxide to produce a composite cylindrical blank having a silver-cadmium oxide core and a malleable silver-cadmium alloy external surface sheathing, abrading the surface of the sheathed blank at right angles to the length thereof, subjecting the thus treated blank to axial pressure to form the same into a contact rivet, the working surface of the head of which rivet is constituted principally of silver-cadmium oxide, and heating said rivet at a temperature of from 600 to 850 C. for a period of from 5 to 240 minutes in the presence of atmospheric oxygen to oxidize at least the portion of said silver-cadmium alloy sheathing on said working surface to silver-cadmium oxide.

8. The method as defined in claim 7, in which the cylindrical silver-cadmium oxide core is constituted of a References Cited by the Examiner UNITED STATES PATENTS 2,255,120 9/1941 Kiefer et al. 29--155.55 2,360,522 10/1944 Shobert et a1. 29155.55 2,504,906 4/1950 Tremblay 29155.55 3,191,272 6/1965 Gwyn 29l55.55

JOHN F. CAMPBELL, Primary Examiner.

P. M. COHEN, Assistant Examiner. 

1. OF A METHOD OF PRODUCING A SILVER-CADMINUM OXIDE ELECTRICAL CONTACT ELEMENT, COMPRISING FORMING A MALLEABLE EXTERNAL SURFACE SHEATHING CONSTITUTED OF A SILVER-CODMIUM ALLOY ON A SILVER-ADIUM OXIDE CORE, COLD WORKING THE RESULTING BLANK INTO THE CONFIGURATION OF THE DESIRED CONTACT ELEMENT AND OXIDIZING AT LEAST THE PORTION OF SAID SHEATHING ON THE WORKING SURFACE OF THE CONTACT ELEMENT TO SILVER-CADMIUM OXIDE. 